An optical isolator is a passive optical component having a pair of incident and emerging terminals. Typically, the optical isolator has low-loss characteristics with regard to forward direction light propagating from the incident side to the emerging side and high-loss characteristics with regard to backward direction light returning from the emerging side to the incident side, thus allowing the passage of the light only in a prescribed direction.
Such an optical isolator may include an optical system of a Faraday rotator configured to rotate a plane of polarization by 45°, the Faraday rotator being disposed between two polarizers having transmission axes displaced by 45° from each other. In this case, a polarized component of the forward direction light that is extracted by the polarizer on the incident side has its polarization direction rotated by +45° by the Faraday rotator, so that the polarized component corresponding to the direction of the polarizer on the emerging side can emerge from the optical isolator without attenuation. On the other hand, in the case of the backward direction light, the polarized component extracted by the polarizer on the emerging side has its polarization direction rotated by −45° by a nonreciprocal action of the Faraday rotator, so that the polarized component is perpendicular to the direction of the polarizer on the incident side. As a result, the backward direction light is attenuated and does not emerge from the incident side. This type of optical isolator in which the polarization direction of incident light is made to correspond to the polarization direction of the polarizer on the incident side in advance may be referred to as a polarization-dependent optical isolator.
Examples of the material of the polarizers in this type of optical isolator include a prism of a birefringent single-crystal, glass containing metal particles, and a composite multilayer film of dielectric and metal materials. For example, Japanese Laid-Open Patent Application No. 2006-113360 (Patent Document 1) discloses an optical terminal having an optical isolator. The optical isolator includes an incident-side polarizer, a magnetic garnet thick film, and an emerging-side polarizer, which are layered. The incident-side polarizer may comprise the Polarcor™ glass material having metal particle diffusion layers formed on both its sides. The emerging-side polarizer may also comprise the Polarcor™ glass material, with the metal particle diffusion layer on the emerging side having been removed (by polishing one side of a standard Polarcor™ product, which may have a thickness of 0.5 mm, down to a thickness of 0.2 mm, for example). It is discussed in Patent Document 1 that a permanent magnet 4 applies a magnetic field that causes the magnetic garnet thick film 12 to function as a Faraday rotator by which the plane of polarization of an optical signal is rotated, whereby the incident-side polarizer and the emerging-side polarizer pass polarized light of only a specific direction.
The materials used in the polarizer discussed in Patent Document 1 are expensive, and its manufacture requires various processing steps including cutting and optical polishing, which tend to make it difficult to reduce manufacturing cost as well as the cost of the optical isolator itself. In fact, more than half of the manufacturing cost of a conventional optical isolator may be due to the cost of polarizers.
In order to overcome the aforementioned problem, Japanese Laid-Open Patent Application No. 2000-180789 (Patent Document 2) discloses an optical isolator in which a reflecting-type first polarizer comprising a photonic crystal, a light-transmitting parallel-plate glass, a parallel-plate 45° Faraday rotator, and a reflecting-type second polarizer of a photonic crystal are arranged and fixed in parallel. One problem associated with this optical isolator is that the entire optical system needs to be disposed at an angle with the optical axis of incident light because of the use of the reflecting-type polarizers in the optical isolator. Such an inclined arrangement of the optical system results in a complex optical system layout.